The use of solar energy to supplement water heating systems is becoming increasingly practical with developments in technology. In Australian Pat. No. 509901 there is described a solar boosted heat pump system which converts solar energy particularly efficiently and transfers that energy by the use of a refrigerant heat pump.
In developing a solar boosted heat pump system for the heating of domestic water, the inventors tested various heat exchange systems for transferring the heat from the refrigerant circulated by the heat pump to the water to be heated. One such arrangement included the immersion of a finned tube carrying the refrigerant in a tank containing the water to be heated. This arrangement was thought to provide the most efficient means of transferring heat from the refrigerant to the water to be heated even though the system was expensive to manufacture. However, testing of the system showed that it was impractical due to the fact that the spaces between the fins tended to fill with the sediments commonly found in some domestic water supplies. Although attempts were made to redesign the finned tube to avoid the collection of sediments, the problem could not be overcome.
A number of attempts to provide heat pump water heaters may be found in the patent literature. For example, U.S. Pat. Nos. 2,716,866 Silva and 4,452,050 Pierce, and United Kingdom Patent No. 1,466,980 Cromarty, each show water heaters having a refrigerant carrying tube surrounding a heat exchanger tank containing water to be heated. In each of the above examples, the heat exchange system is both complex and expensive due to the use of a number of separate windings (or winding sections, in the case of Cromarty) aimed at extracting the higher temperature superheat from the refrigerant fluid carried by the tubes. The use of separate windings or winding sections increases the material and manufacturing costs of the heat exchanger, and the inventors believe that these costs are not justified by the benefit which may be achieved by extracting the superheat from the refrigerant fluid. Furthermore, none of the systems described in the above literature envisages use of the system in association with a solar evaporator.
Of course water heating systems utilising solar energy are known, and one such system is described in U.S. Pat. No. 4,282,861 Roark. However, this system uses the water to be heated as the heat transfer medium and such a system does not have the required efficiency to function adequately as a domestic or industrial water heating system.
The patent literature also contains numerous examples of heat exchange systems in which pipes are attached to tanks, such as United Kingdom Patent No. 377398 Bishop et al and German patent application No. P3325137 Knabben et al, but these disclosures are not directed towards heat pump systems in which the heat transfer fluid is a refrigerant or the like. For example, the Bishop et al patent pipes steam around a cooking pan and as such is impractical for application to a domestic water heater. In the case of Knabben, the invention is concerned with the shaping of the tube to maximize contact between the tube and the heat exchange tank, and the heat exchange medium is not even discussed. Thus, while patents such as the above disclose the attachment of heat exchange tubing to the lower portion of a liquid containing tank, the earlier mentioned patents directed to systems using refrigerant as the heat transfer medium would suggest that such a limited use of exchange tubing would not be satisfactory for a system using a heat pump.